The present invention relates to a magneto-optical recording medium capable of enlarging recorded magnetic domains for reading purposes.
In a magneto-optical recording medium, a magnetic thin film thereof is locally heated by laser beam irradiation or other light irradiation to decrease its coercive force. Then, an external magnetic field is applied to the magnetic thin film to reverse the direction of magnetization of the irradiated spots or maintain the initial magnetization direction, thereby forming magnetic domains (recorded marks). The direction of magnetization of the magnetic domains is read out by the Kerr or Faraday effect.
In an ordinary magneto-optical recording medium, the available density of readable magnetic domains is limited by the spot diameter of the laser beam used for reading, and so it is impossible to read magnetic domains having a diameter smaller than the half of the laser beam spot diameter.
For a magneto-optical recording medium capable of reading magnetic domains having a diameter smaller than the half of laser beam spot diameter, for instance, JP-A 8-7350 discloses a magneto-optical recording medium capable of enlarging recorded magnetic domains. This magneto-optical recording medium comprises a triple-layered recording film comprising, in order from an substrate side, a first magnetic layer, a second magnetic layer, and a third magnetic layer which are exchange coupled together. Upon reading, the substrate of the medium is irradiated with a laser beam while a reading magnetic field is applied to the recording film of the medium. The third magnetic layer holds recorded magnetic domains, which are copied by laser beam irradiation to the second, and first magnetic layers. The copied magnetic domains are enlarged by the application of a reading magnetic field in the longitudinal direction of each magnetic layer. The enlarged, copied magnetic domains are read out as is the case with a conventional magneto-optical recording medium. After the reading of the enlarged, copied magnetic domains has finished, an erasing magnetic field opposite in direction to the reading magnetic field is applied to erase off the copied magnetic domains for the next reading of adjacent recorded magnetic domains. Thus, upon reading of this magneto-optical recording medium, a modulated magnetic field comprising a reading magnetic field and an erasing magnetic field is applied thereto. By repetition of such a process, it is possible to read minute magnetic domains unreadable so far in the art. Furthermore, this method does not only achieve high resolution upon reading, but also enables output signal intensity to be in itself augmented because the magnetic domains are actually enlarged. Hereinafter, the reading process capable of reading minute magnetic domains by enlargement of the copied magnetic domains will be called a reading process by enlargement of magnetic domains.
In the magneto-optical recording medium set forth in the aforesaid publication, however, the magnetic film of an exchange coupled multilayer structure is used. In other words, to enlarge copied magnetic domains, it is required to overcome exchange force between adjacent magnetic layers; it is required to use a large reading magnetic field. To achieve recording density, and transfer rate increases, it is required to allow a modulated magnetic field comprising a reading magnetic field and an erasing magnetic field to have a high frequency. However, the larger the magnetic field generated by a modulated magnetic field generating means, the more difficult is it to allow the modulated magnetic field generating means to have a high frequency. Thus, the use of the exchange coupled magnetic film is an obstacle to achieving recording density, and transfer rate improvements.